Question

In crystallization, the ............... is defined as the region between the solubility and super solubility lines.

• A. Metastable zone width
• B. Metastable zone
• C. Metastable region
• D. Labile region

Hint:

Standard Crystallization Zones:
1. Stable (Unsaturated): Dissolves crystals.
2. Metastable: Growth only (needs seeds).
3. Labile (Unstable): Spontaneous new nuclei form.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the name of the phase region in a crystallization process where the solution is supersaturated but will not spontaneously crystallize without a stimulus (like seeding).

Step 2: Detailed Explanation:

• Crystallization involves three primary zones of saturation: unsaturated, supersaturated, and supersolubility.

• The Solubility line represents the thermodynamic equilibrium where the solution is saturated. No more solute can dissolve, but no crystals form.

• The Super solubility line represents the point of maximum supersaturation beyond which spontaneous (primary) nucleation occurs. This region is called the Labile region.

• The region located between these two lines is known as the Metastable zone.

• In the metastable zone, the solution is supersaturated, but the system is relatively stable. Spontaneous nucleation will not occur here. However, if "seeds" (existing crystals) are added, they will grow.

• Controlling the metastable zone width is crucial in industrial crystallization to ensure the production of uniform, large crystals without generating unwanted "fines" (tiny new nuclei).

• Options (A) and (C) are descriptive, but "Metastable zone" (B) is the standard technical term for the region itself.

Step 3: Final Answer: The region between the solubility and super solubility lines is the Metastable zone.